1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device having aluminum electrodes with a semi-insulating film formed thereon.
2. Background Art
Semiconductor devices employing Al electrodes have been widely used. In such a semiconductor device, some of these Al electrodes serve as gate and emitter electrodes in the device region and others are formed on guard rings in the guard ring region formed around the device region. A guard ring is a P-type region formed within an N-type substrate to reduce the electric field. As is known in the art, a semi-insulating film may be formed over the Al electrodes to stabilize their potentials.
In such a case, the semi-insulating film continuously extends over the separate Al electrodes formed on the respective guard rings. More specifically, the semi-insulating film covers the guard rings and the spaces therebetween and has the following effects. The formation of the semi-insulating film between the guard rings means that the guard rings are connected to one another through a high resistance, that is, they are not fully insulated from each other. This makes the potentials of the guard rings substantially equal, thereby increasing the dielectric strength of the device. Furthermore, hot electrons trapped in the guard rings leave the rings and enter the semi-insulating film, thereby stabilizing the potentials of the guard rings.
The edge termination dimensions (or width) of the semiconductor device can be reduced by taking advantage of the increase in the dielectric strength of the device, the stabilization of the potentials of the guard rings, etc. which result from the use of the semi-insulating film. More specifically, since the use of the semi-insulating film ensures that the device has sufficient dielectric strength, a dielectric strength enhancing region thereof called “edge termination” can be narrowed. Further, forming a semi-insulating film in contact with the gate and emitter electrodes (i.e., electrodes in the device region) results in stabilizing their potentials.
Techniques for enhancing the dielectric strength of a semiconductor device, such as described above, are disclosed, e.g., in the following patent publications. Japanese Laid-Open Patent Publication No. 04-212468 (1992) discloses a semiconductor device in which a low resistance film is disposed over field plates to increase the dielectric strength of the device. Japanese Laid-Open Patent Publication No. 2000-183366 discloses a semiconductor device in which a semi-insulating passivation film is disposed over field limiting rings (or guard rings) so that the rings receive equal intensities of electric field resulting in increased dielectric strength of the device. Further, Japanese Laid-Open Patent Publication No. 06-275852 (1994) discloses a semiconductor device in which the Al electrodes on guard rings and on channel stoppers are in contact with a semi-insulating film.
As described above, the Al electrodes in the device region and guard ring region of the semiconductor device may be covered with a semi-insulating film to stabilize the potentials of these electrodes and enhance the dielectric strength of the device. However, this arrangement is disadvantageous in that in the manufacture of the device, the Al electrodes may chemically react with Si contained in the semi-insulating film due to the heat generated when forming the semi-insulating film or due to the heating of the semiconductor substrate at a subsequent step. In the past this reaction has caused such problems as increased contact resistance between the Al electrodes and the semi-insulating film, poor appearance, reduced resistance between isolated Al lines (which may ultimately lead to shorting therebetween), and degraded reliability of the device. It should be noted that not only the guard ring region but also the device region can suffer these problems, since the above chemical reaction may occur wherever an Al electrode is in contact with a semi-insulating film.